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  DB<122> $a
 => 5
...

  DB<125> $a
 => 7

  DB<126> sub ref_add { ${ shift() }+=2 }

...

  DB<131> $a
 => 11

  DB<102> $a=11
 => 11
...

  DB<104> ref_add3 \$a; $a
 => 13

       Any arguments passed in show up in the array @_.  Therefore, if
+ you
       called a function with two arguments, those would be stored in 
+$_[0]
...
       for the actual scalar parameters.  In particular, if an element
+ $_[0]
       is updated, the corresponding argument is updated (or an error 
+occurs
       if it is not updatable).